In the knitting industry, for instance, and in other fields of the textile industry as well, it is often necessary to supply knitting stations or other yarn-using stations with defined quantities of textile yarns. So-called positive feed mechanisms are used for this, which allocate the requisite yarn quantity to the respective yarn-using station and deliver it, regardless of the actual yarn takeup at the yarn using station. One such feed mechanism is known, for instance, from German Patent DE 3601586 C1. This feed mechanism is intended in particular for circular knitting machines, which have knitting stations with constant yarn consumption. The feed mechanism has a shaft, rotatably supported in a basic carrier, on one end of which a yarn feed drum is held in a manner fixed against relative rotation. The other end of the shaft is provided with a pulley that is in engagement with a toothed belt. This toothed belt is a driven belt which travels over the pulleys of many such yarn feeders that are thus driven at the same constant rpm.
The yarn feed drum has an outer circumference which is defined by many bars extending in the axial direction. The bars are held on their ends in terminal disks that are embodied substantially conically on their side toward the bars. The yarn to be fed wraps multiple times around the yarn feed drum, so that the drum draws the yarn from a bobbin and feeds it to the knitting station. From the yarn drum, the yarn is guided through eyelets which thus define a yarn travel path. The eyelet that follows the yarn feed drum is offset axially from the yarn feed drum so far that the yarn sweeps over the edge of the yarn feed drum. This aids in stripping off fluff from the edge of the drum and can be favorable to the takeup of additional yarn windings, if the yarn sags or begins to sag because there is no yarn takeup, or only slight yarn takeup, at the knitting station while feeding continues at a constant pace. The yarn then sags over the edge and continues to rest on the drum.
Yarn feeders of this type are highly suitable for applications involving substantially constant yarn consumption. However, applications also exist in which yarn consumption can be subject to very major fluctuations temporarily. This is the case, for instance, in flatbed knitting machines, in which a yarn guide, that delivers the yarn to the individual needles in succession, executes a reciprocating motion of the full width of the goods being produced. If the yarn feeder is disposed laterally, that is, if the yarn is fed crosswise to the width of the goods, the yarn consumption differs on the forward and return strokes. Furthermore, yarn consumption comes to a complete stop at the turning points of the yarn guide. In these cases, yarn feeders that feed yarn at a constant quantity over time certainly cannot be used. On the contrary, it is necessary that the fed yarn have a substantially constant tension, so that a constant mesh size will be obtained. To that end, a yarn feeder that has a motor-driven yarn feed wheel is known from German Patent Disclosure DE 19537215 A1. The yarn wraps several times around this wheel. After that, the yarn runs over a yarn tension sensor to the yarn-using station. The yarn tension sensor regulates the rpm of the motor of the yarn feed wheel via a closed control loop, so that the yarn tension is regulated to the set-point value.
The yarn feed wheel is formed by six vanes or wire brackets extending away from a central hub. The vanes or brackets each have two spaced-apart, parallel, radially extending legs, between which an axially oriented bearing portion is held. The bearing portion changes over into the radial spokes via radially outward-protruding portions bent into a U.
The payout direction of the yarn from the yarn feed wheel is, as for instance in German Patent Disclosure DE 4206607, defined at right angles to the pivot axis of the yarn feed wheel.
In the yarn feeder of DE 4206607 A1, a yarn feeder with intersecting bearing struts for the yarn is used. The yarn feed wheel thus has its smallest outside diameter in its center plane, so that a yarn delivered eccentrically slides onto this middle position. From there, it is drawn off via a yarn directing means, embodied here as a helical spring wound in the shape of a trumpet, in the center plane of the yarn feed wheel.
Such yarn feeders are intended for feeding elastic yarns. Such yarns include, for instance, Spandex.TM. yarns or other kinds of synthetic fibers. Because of their elasticity, it is critically important to monitor the yarn tension. It can also prove problematic that the often very thin (hair-fine) yarns have a tendency to wrap windings on top of one another or to form windings that stick together.
A repeatable and uniform separation over time of the quantities of yarn fed by the yarn feed wheel from windings located on the yarn feed wheel is especially difficult to achieve when rotary speeds are varying. However, a failure of the yarn to be released from the yarn feed wheel at high or otherwise critical rotary speeds so that it is, instead, entrained sticking together must be prevented, because that would lead to reverse feeding and thus cause the yarn to tear.